In devices running with their natural frequency, such as free piston machines, their frequency or cyclic speed is a function of the energy input portion and energy output portion of the device, such as the power piston energy input portion and compressor piston energy output portion of a free piston engine compressor. In other words the speed of such an engine compressor will vary in a given fixed natural relationship with variations of compressor volumetric flow and compressor intake pressures and/or discharge pressures. Thus, in a heat pump, for example, a greater volumetric flow rate is typically required at the lower intake and discharge pressures involved in operating at low ambient temperatures. However, even at maximum piston stroke, not only is the volumetric output per stroke reduced by the lower intake pressure, but the rate of volumetric flow is further substantially reduced by the lower natural frequency which the engine compressor will assume under the influence of both the lower intake and discharge pressures.
This fixed natural relationship has not heretofore been controlled efficiently over a range wide enough to prevent or overcome, in a practicable manner, the problem just described, and specifically to provide for an improved linear free piston engine capable of driving any one of a number of different types of energy absorbing devices.
In an effort to provide some extent of speed control for machines such as free piston engine compressors, one previous proposal involved a device in which the reciprocating free piston masses had hollow portions which could be filled to varying degrees with a liquid such as oil and, thus, by variation of the reciprocating masses, the speed was to be controlled within a rather limited range, since the cyclic speed varies with the mass according to the equation: ##EQU1## which for a given characteristic engine, can be reduced to: ##EQU2## when F is the driving force, s the stroke and s.sub.1 and s.sub.2 the inner and outer limits of the stroke of m (the reciprocating or driven mass), V the velocity of the mass and n the cyclic speed. As equation II shows, the mass would have to be reduced for instance to one quarter to obtain twice the cyclic speed which, in practice, would be a most difficult and most likely impractical result to achieve.
In connection with the use of free piston engine gas generators (gasifiers) for supplying exhaust gas to drive a turbine, prior efforts have been made, as in Lewis, U.S. Pat. No. 2,435,970, to control the frequency of a free piston engine by means of an additional bounce chamber in conjunction with a bounce chamber on the back side of a load compressor piston to seek lower and higher frequency limits than would be possible without such an additional bounce chamber. While Lewis suggested that his desired operation could be accomplished with the help of governors responsive to various operating conditions to adjust or respond to pressures in a number of engine locations, including direct (positive) and reverse (negative) bounce chambers and other portions of his gasifier, the Lewis patent does not disclose an engine and control combination capable of meeting the problems recognized and solved by the present inventor in the manner described in this application.